DOE OSTI2016
Nuclear energy has been an important source of power throughout the world for several decades, but with that power comes substantial amounts of waste. The USA alone is projected to produce 77,100 MTHM (34,700 MCi) of commercial spent fuel by the year 2020. There has been a growing sense of urgency to determine a method of disposal for all types of nuclear waste. Underground storage has been proposed as the safest means of long term storage and disposal by both the Academy of Sciences and the Blue Ribbon Commission with a focus on deep geologic disposal. Because deep geological repositories (DGR) pose as the safest method of disposal due to the isolation of the waste and the low-permeability of the surrounding material, it is crucial to understand the science behind these repositories. Many scientists have concern with DGRs because of all the unanswered scientific questions. With a global inventory of 300,000 MTHM, it is imperative to understand how DGRs will retain the nuclear waste over a time span of up to a projected 1 million years. Because of the amount of nuclear waste increases each year, there is an inherent need to study the viability of a repository in order to begin implementation of that repository. Based on the half-lives of each radionuclide in nuclear waste, after approximately 100 years of storage, the heat production from the decay of radionuclides becomes dominated by the long-lived actinides. Because the decay of the nuclear waste results in an elevated temperature within the canister, That mixed with water intrusion can corrode the canister and allow radionuclides to leach into the surrounding environment. In an effort to retain these radionuclides within the repositories, most all deep geologic repository designs incorporate an engineered multi-barrier approach. Once the radionuclides begin to leach out of the canisters, the next barrier to retain them is a layer of compacted clay such as a smectite. A mechanistic understanding of mobility at elevated temperatures is crucial in order to evaluate the performance of the engineered clay barrier system within a deep geological repository. Because the clay barrier is compacted so tightly, the only method of mobility for radionuclides is through molecular diffusion. Assessing the factors that affect the rates at which various radionuclides will diffuse through these clays is important in the performance assessment of the repository system as a whole. Neptunium-237 has been identified as a major actinide of concern for long term storage, dominating the radioactivity emitted from the waste between 10,000 to 100,000 years of storage. Under oxidizing conditions, this radionuclide exists mainly as Np(V). In this oxidation state, the neptunium exists chemically as the NpO{sub 2}{sup +} ion which exhibits enhanced mobility characteristics such as high solubility and low affinity for sorption to surfaces. Because of this, {sup 237}Np(V) has been identified as the most mobile radionuclide in nuclear waste and thus a major radionuclide of concern, motivating research into the mobility of {sup 237}Np(V) through the compacted clay barrier. Bentonite is one of the most common backfill materials used in repositories. This clayey rock's main constituent is the mineral Na-montmorillonite [(Ca{sub 0.06}Na{sub 0.14}K{sub 0.05})(Al{sub 1.57}Fe{sup 3+} {sub 0.17}Mg{sub 0.25})(Al{sub 0.04}Si{sub 3.96}O{sub 10} - (OH){sub 2})] which gives this material the advantageous properties required for a repository such as a high swelling capacity. Because of this high affinity for absorbing water, montmorillonite is the ideal candidate for backfill material in a deep geological repository. Adsorption of radionuclides onto the surface of the compacted clay is a major mechanism for retardation of mobility. Because diffusion and adsorption are integrated into this process, it is important to characterize Np adsorption onto the surface of compacted montmorillonite. Previous studies have quantified the adsorption of Np onto montmorillonite at variable ionic strengths and temperatures. Furthermore, several studies have focused on the diffusion of Np through compacted clay, but no other studies have determined diffusion coefficients at elevated temperatures of up to 80 deg. C. (authors)
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗